Does membership guarantee a ticket?

No. It only unlocks an earlier window. Inventory can still sell out inside that window.

Can I join membership on sale day?

Usually too late — notices set a join-by date before the first presale.

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K-pop membership & concert presale windows explained

Membership / fan-club presale — paid members enter first with a code or registered account.
Partner / card / Verified Fan — optional middle windows run by promoters or banks.
Global or general sale — remaining seats open to anyone the platform accepts.